(1) Field of the Invention
The present invention relates to a treated surface coat of a copper foil and more particularly to a copper foil for printed circuits which provides a heat-discoloration resistant and rust-proofing coat, and also a method of producing same.
(2) Description of the Prior Art
A printed circuit board is progressing rapidly nowadays and it is required to grade up its reliability and high efficiency. Of course, such requirements become more and more serious in the case of a copper foil for printed circuits.
The first step of producing a printed circuit board is to laminate a copper foil on a resin impregnated base material under heat and pressure so as to obtain a copper-clad laminate. In this case, where paper-phenolic resin for consumer use is used as the material of the board, the temperature of pressing is set to be 150.degree..about.160.degree. C. However, where glass-epoxy resin for industrial use is set to be 10.degree..about.20.degree. C. higher in comparison with the temperature in the case of the paper-phenolic resin. Further, where plastics of high heat resistance or glass-polyimide resin, which is used increasingly nowadays, is used as the material of the board, it requires to set the temperature higher.
By the way, according to high density or high multiplication of the printed circuit board, it is required to carry out postcure after pressing process in order to minimize a rate of change of size thereof.
Therefore, the shiny side surface of the copper foil, which is not the side of the copper foil to contact with the base material, is normally affected with oxidation due to heat for a long time aging of the postcure with a high temperature, and if the shiny side surface is not treated, discoloration due to oxidation occurs strongly. As the result, no value is obtained by the copper foil in both its surface characteristics and its appearances.
Accordingly, Japanese patent publication No. 54-29187, for example, discloses a method of forming a coat by immersing a copper in an alkaline aqueous solution including zinc or by turning on an electric current to the copper as an anode in the aqueous solution. Japanese patent publication No. 58-7077, discloses a method of providing a coating layer of zinc or a compound which consists of zinc or zinc oxide and an oxide of chromium. Further, U.S. Pat. No. 4,082,591, discloses a method of providing a coating layer of copper-tin alloy. Further, Japanese patent laid-open publication No. 57-152490, discloses a method of electrolyzing a copper foil as a cathode in a solution which includes a hexavalent chrome compound, a water soluble zinc compound and sodium sulfate, and PH of which is set less than 6.5.
However, the above mentioned conventional methods have the following problems. In the case of deposited coat of chrome and zinc compounds or a copper-tin alloy coat, there is no heat resistance against a high temperature and a long time such as at 200.degree. C. for 1 hour, or no heat resistance against postcure after lamination with a base material. Further, it is not sufficient in rust-preventing effects against oxidation in a change with the passage of time in keeping such coats until they are put in laminating processes.
Further, in the method of plating zinc, when an amount of zinc is increased, heat-discoloration resistance may be improved. However, when a great amount of zinc is deposited, its appearance presents a gray color and after lamination under pressure the color resembles gold, and the solderability becomes poor and its market value is lost.